The critical role of aqueous-phase processes in aromatic-derived nitrogen-containing organic aerosol formation in cities with different energy consumption patterns

Abstract. Nitrogen-containing organic compounds (NOCs) impact air quality and human health. Here, the abundance, potential precursors, and main formation mechanisms of NOCs in PM2.5 during winter were compared for the first time among Haerbin (dependent on coal for heating), Beijing (natural gas and coal as heating energy), and Hangzhou (no centralized heating policy). The total signal intensity of CHON+, CHN+, and CHON− compounds was highest in Haerbin and lowest in Hangzhou. Anthropogenic aromatics accounted for 73 %–93 % of all identified precursors of CHON+, CHN+, and CHON− compounds in Haerbin. Although the abundance of aromatic-derived NOCs was lower in Beijing than in Haerbin, aromatics were also the main contributors to NOC formation in Beijing. Hangzhou exhibited the lowest levels of aromatic precursors. Furthermore, non-metric multidimensional scaling analysis indicated an overall reduction in the impact of fossil fuel combustion on NOC pollution along the route from Haerbin to Beijing to Hangzhou. We found that aqueous-phase processes (mainly condensation, hydrolysis, or dehydration processes for reduced NOCs and mainly oxidization or hydrolysis processes for oxidized NOCs) can promote the transformation of precursors to produce NOCs, leading to the most significant increase in aromatic NOC levels in Haerbin (particularly on haze days). Reduced precursor emissions in Beijing and Hangzhou (the lowest) constrained the aqueous-phase formation of NOCs. The overall results suggest that the aerosol NOC pollution in coal-dependent cities is mainly controlled by anthropogenic aromatics and aqueous-phase processes. Thus, without effective emission controls, the formation of NOCs through aqueous-phase processes may still pose a large threat to air quality.